Throughout this specification including the claims, the terms "post-decision feedback equalization noise" and "post-DFE noise" denote noise components of signals undergoing decision feedback equalization (before or after the decision feedback equalization operation has converged).
Also throughout this specification including in the claims, the terms "symbol" and "data symbol" are used to denote any characteristic of a signal. Examples of data symbols include amplitude of a digital voltage signal (whose amplitude can have any of a set of discrete values), phase of an electrical current signal, or an amplitude of a Fourier component of an electrical voltage pulse.
An inherent problem with transmission of data over a communication channel is that distortion and additive noise tend to interfere with proper reception of the transmitted data. Distortion during transmission of data pulses alters the received data symbols (e.g., the received pulse shape of each data pulse). This causes each symbol to interfere with several adjacent symbols, which inhibits the receiver from performing symbol detection and timing recovery. Additive noise further degrades the ability of the receiver to distinguish between received symbol levels.
Conventional receiver filtering techniques can counter distortion and additive noise effects to provide good symbol decision capability. For example, one type of conventional receiver (shown in FIG. 1) includes a linear feedforward filter followed by a nonlinear decision feedback equalizer (DFE). Conventional DFE circuits are nonlinear, due to quantizer circuitry therein which performs symbol decisions in the DFE feedback loop.
In the conventional circuit of FIG. 1, input signal s(n) has a z-domain representation s(z) (the parameter "n" can represent time, and "z" can represent frequency). Input signal coefficients s(z) propagate through the transmission channel identified as filter 2, which has z-domain transfer function H(z). Filter 2 typically has unknown characteristics. After propagating through the transmission channel (i.e., after being filtered by filter 2), the input signal is filtered by receive filter 4, which has z-domain transfer function R(z). As indicated in FIG. 1, the combined transfer function of filter 2 and receive filter 4 is A(z), and coefficients "t(z)" are the z-domain response of the combination of filters 2 and 4 to input coefficients s(z).
Additive noise (identified as "u(z)" in FIG. 1) typically becomes associated with the input signal during propagation through the transmission channel. To reflect this phenomenon, FIG. 1 indicates the presence of noise u(z) at the input of receive filter 4, and identifies the response of filter 4 to additive noise u(z) as filtered noise "w(z)." The combined response of filter 4 to noise u(z) and to filter 2's response to signal s(z) is identified as "x(z)" in FIG. 1. Combined response x(z) undergoes further processing (decision feedback equalization) in DFE components 8, 10, 12, and 14 of the FIG. 1 apparatus. Although FIG. 1 represents generation of combined response x(z) by summation of filtered noise w(z) with response t(z), it should be appreciated that an actual physical implementation of the FIG. 1 apparatus would include a single filter 4 whose single output x(z) has components w(z) and t(z), and that an actual physical implementation of the FIG. 1 apparatus would not include an actual summation circuit for summing together two distinct signals (corresponding to w(z) and t(z)) to generate response x(z).
As indicated in FIG. 1, the decision feedback equalization of response x(z) includes the steps of processing response x(z) in subtraction circuit 8, followed by processing of the output of circuit 8 in quantization circuit 10 and subtraction circuit 14, and generation of feedback coefficients x'(z) in filter 12 for subtraction from response coefficients x(z) in subtraction circuit 8.
To make the following description more definite, input signal s(n) is assumed to be a pulse whose amplitude can have only certain discrete values, i.e., the amplitude of s(n) is a member of a set of L discrete values Q={q.sub.1, q.sub.2, . . . , q.sub.L } for each value of "n." In response to such an input signal, the output signal x(n), having z-domain coefficients x(z), has the following form: ##EQU1## where the first summation represents signal t(n) (whose z-domain representation is t(z)), the second summation represents noise w(n) (whose z-domain representation is w(z)), M is the number of z-domain coefficients of transfer function H(z) of filter 2, and P is the number of z-domain coefficients of transfer function R(z) of receive filter 4. The first coefficient a.sub.0 s(n) of the first summation is indicative of input signal s(n). The other coefficients of the first summation represent intersymbol interference. Coefficients r.sub.k of receive filter 4 can be determined adaptively, or by some fixed criteria that determine one or more pulse shape characteristics.
The function of DFE loop components 8, 10, 12, and 14 of FIG. 1 is to cancel the inter-symbol interference and perform symbol detection (to generate a replica s'(n) of input signal s(n)). Practical embodiments of this DFE circuitry will equalize the lowest N z-domain coefficients of signal x(n), where N&lt;M+P, while approximating the first N of above-mentioned coefficients a.sub.j. In such embodiments, the DFE circuitry can equalize part but not all of the response t(z) to combined filters 2 and 4. After the DFE circuitry has converged to a final version of replica signal s'(n), such final version of signal s'(n) will satisfy the following relationship: s'(n)-a.sub.0 s(n)=e(n)=w(n). In other words, the final version of signal s'(n) will differ from a scaled version of input signal s(n) by error signal e(n), where e(n) has z-domain coefficients e(z) which satisfy e(z)=w(z), where w(z) are the z-domain coefficients of filter 4's response to additive noise u(n). Before the DFE circuitry reaches convergence, quantizer 10 outputs (during each feedback iteration) replica coefficients s'(z), which in turn cause circuit 14 to generate error components e(z) of an error signal e(n) which satisfies e(n)=v(n)+(n) where v(n) represents residual inter-symbol interference and w(n) is a filtered additive error signal having z-domain coefficients w(z).
We next describe the operation of conventional DFE circuitry 8, 10, 12, and 14 in greater detail, with reference to FIG. 1.
In subtraction circuit 8, feedback coefficients x'(z) (replicas of coefficients x(z) generated by filter 12 in a manner to be described below) are subtracted from signal x(z) to generate difference coefficients y(z)=x(z)-x'(z). Quantization circuit 10 processes difference coefficients y(z) to generate replica coefficients s'(z) which define a replica signal s'(n) whose value is the member of the set {q.sub.k ; k=1, 2, . . . , L} which best approximates input signal s(n).
Replica coefficients s'(z) are subtracted from coefficients y(z) in subtraction circuit 14 (after coefficients y(z) are multiplied by coefficient a.sub.0 by means within circuit 14), to generate above-mentioned error coefficients e(z). In an embodiment in which filter 12 is an adaptive filter, replica coefficients s'(z) and error coefficients e(z) are fed back to filter 12. In response, filter 12 applies adaptively generated transfer function A'(z) to coefficients x(z), to generate a set of replica coefficients x'(z) during each iteration of the DFE operation. Error coefficients e(z) can thus be used to implement adaptive convergence and continuous coefficient updating within filter 12.
In other embodiments in which filter 12 has a predetermined transfer function A'(z) which matches the transfer function A(z), only coefficients s'(z) are fed back to filter 12, and in response to coefficients s'(z) only, filter 12 generate a set of replica coefficients x'(z) during each iteration of the DFE operation.
The present invention pertains to methods and apparatus for controlling (compensating for) the error coefficients w(z) which remain after the conventional apparatus of FIG. 1 has converged (as mentioned above, e(z)=w(z) at convergence).
Both before and after convergence, the power spectrum and power of a residual error signal generated during DFE processing depends not only on the unequalized channel coefficients v(z) convolved with the symbol alphabet {q.sub.k }, but also on receive-filtered noise coefficients w(z) (which are receive filter 4's response to additive noise u(z)). The inventor has recognized the desirability of controlling the coefficients w(z), primarily due to the following reasons.
A receive filter (such as receive filter 4 of FIG. 1) that is employed to process input data in a decision feedback equalizer will sometimes be referred to herein as a "feedforward" filter. A feedforward filter is typically designed to perform pulse shaping functions such as decreasing the rise time of each input pulse which it receives (for timing recovery purposes), or suppressing the tail of each input pulse which it receives to enable minimization of the number (N) of coefficients of a filter in a DFE feedback loop (e.g., filter 12 of FIG. 1). However, it is usually not possible for a practical feedforward filter to achieve such pulse shaping objectives while also achieving desired noise filtering objectives. For example, a high pass feedforward filter which sharpens the leading edge of a received pulse will also undesirably increase the power of the associated noise (assuming the noise is white noise or high frequency noise).
The present invention permits a feedforward filter (used with a decision feedback equalizer) to be implemented with a simple design for achieving desired pulse shape and length, while also controlling the residual noise present during decision feedback equalization. The invention accomplishes this by filtering the noise components of signals undergoing decision feedback equalization, for example to compensate for noise enhancement that has undesirably resulted from pulse shaping by the feedforward filter.